Focus: Turbulence Helps Photons Remain Connected

The turbulent atmosphere is better than an equivalent glass fiber when it comes to preserving quantum information in long-distance light signals, according to a new analysis. For brief moments, light can propagate relatively undisturbed.

From a Distance. Experiments last year found that pairs of photons remained quantum-mechanically connected even as they traveled 144 kilometers between the Canary Islands of La Palma (foreground) and Tenerife (center). A new analysis says this durability reflects the anomalously good conditions seen by the few photons that survive the trip.

Light signals sent through the air are constantly jostled and diverted by turbulence in the atmosphere. But in the February Physical Review A researchers suggest that quantum-mechanical information encoded in light passes better through air, during fleeting moments when conditions are ideal, than through an equivalent glass fiber. The team explains experiments from last year showing that pairs of photons can remain quantum-mechanically intertwined over more than 100 kilometers. The results raise hopes for eavesdropping-proof communication that exploits these connected pairs of photons.

Encrypted data is critical to secure financial, military, and personal communication. Some researchers expect that future ultrafast computing based on quantum mechanics could crack current coding schemes. But quantum mechanics could also help solve the problem by allowing the safe distribution of cryptographic keys–the strings of numbers that allow encrypted data to be read. A key could be sent using “entangled” pairs of photons for which a measurement on one automatically affects the results for a measurement on the other. This delicate quantum connection between the pair would be disrupted if the photons were intercepted, alerting the key sharers to the eavesdropper’s presence.

This scheme would fall short, however, if entanglement is lost as photons travel through the turbulent atmosphere. But last year a group led by Anton Zeilinger of the Austrian Academy of Sciences and the University of Vienna detected pairs of photons that remained entangled even after travelling for half a millisecond between two of the Canary Islands, off the coast of Morocco [1]. Now Andrew Semenov of the Institute of Physics in Kiev, Ukraine, and Werner Vogel of the University of Rostock, Germany, say that the persistent entanglement of those pairs that survive the trip occurs in part because they had enjoyed unusually clear sailing through the atmosphere.

The experimenters created pairs of entangled photons on one island and shined them together to a telescope 144 kilometers away on another island. The team detected both photons for fewer than one in a million starting pairs, in part because of turbulence, but they found a high degree of entanglement for those pairs. The need to see both photons, Semenov says, is a kind of “post-selection” and means that the experiment only tests pairs that happened to come across during a moment when conditions were best. “This is not a special property of the atmosphere,” Semenov says; it’s a “special property of a post-selection measurement.” Semenov and Vogel calculated that these successful pairs would retain a high degree of entanglement, in spite of the turbulence.

But this doesn’t mean that even more turbulence would improve quantum transmission, Semenov cautions. “When the weather is worse, communication will also be worse,” because it will degrade the average transmission. But compared with an optical fiber with similar transmission losses, on average, the fluctuations mean that “the atmospheric channel is better,” says Semenov.

“For us the paper is great news, because it tells us that atmospheric turbulence can work for us. I did not expect that,” Zeilinger says. “It’s good news for long distance quantum communication in the atmosphere in general.”